[TRLog] MFJ-434 Interface with TR-Log

Tom Delker k1ky@bellsouth.net
Thu, 08 Mar 2001 00:26:08 -0600


Thanks to Tree, with version 6.56 released early December, TR-Log will
now properly key the MFJ-434 Voice Keyer. The problem I found with prior
versions was that the keying pulse was too short for the MFJ unit to
recognize. I verified this deficiency in the 434 by using a relay
interface to be sure the input pin on the 434 was being brought entirely
to ground, and the unit absolutely refused to trigger, until a longer
keying pulse was supplied. 

An inverting interface is required to take the +5 positive-going pulse
from the PC parallel port to supply a logic "low" to the voice keyer
inputs. The transistor inverter circuits described in the MFJ manual
should work well. 

In the interest of isolation between the PC and the voice keyer, I chose
to use opto-isolator circuits for the inverters. Using 4N25
opto-isolators in place of the transistors, I used 1K ohm current
limiting resistors between the PC keying pins (DVK 1-4) and the LED
input to the 4N25 IC's. One exception was the "abort" circuit logic
line, which required a 220 ohm resistor instead of the 1K ohm resistor.
This was needed to pull the DVK abort line input low enough to allow the
canceling of DVK port 3. Strangely enough, a 1K was sufficient to cancel
channels 1,2 and 4. 

An interesting note is that the Escape "cancel" key supplies only one
pulse and will not key again until another DVK function key is pressed.
If you are running "auto CQ", you hit ESC once to cancel the automatic
repeat (does NOT send a pulse to the DVK), and hit the ESC button again
to send the cancellation pulse to the voice keyer. Any further punching
of the ESC key will not supply another pulse, until another DVK funtion
is activated or the auto-CQ funtion is re-activated. 

Another interesting note is that although DVK functions 1-4 supply a
single pulse, if you hold the function key down for a length of time,
the PC will supply a continuous voltage to the appropriate DVK port
until the key is released. In the case of the MFJ-434, this will
function the same as holding the button on the keyer and will trigger
the auto-repeat function built into the MFJ unit. 

Having owned the previous model of MFJ Voice keyer, and choosing to use
laptop computers for contesting, I am very impressed with the
improvements and highly recommend the 434 as one of the few pieces of
MFJ equipment worth having that actually performs as advertised without
any weird glitches.   

Many thanks again to Tree for modifying TR at my request to lengthen the
DVK keying pulse in order to make this interface function for this piece
of equipment.

TR(ee) is TOPS!  TR-Forever!

-- 
Thomas M. Delker - K1KY CFO#395 - Smyrna / Gallatin / Short Mtn. TN
A.R.R.L. District Emergency Coordinator - Tennessee District #4
C.A.T.S. (W4CAT) - 147.30/145.23/145.49/145.17/223.98/444.45/444.65
Stones River A.R.C. (K4FUN) - http://www.members.home.com/srarc
Tennessee Contest Group (K4TCG) - http://www.k4ro.net/tcg.html
         CODE WARRIOR   #2  - Mobile CW Operator
     "Putting code on the road using the primary mode"

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